During fabrication of semiconductor integrated circuits, a semiconductor integrated circuit die (backside thereof) is attached to a die attach paddle of a leadframe. Then bond pads of the semiconductor integrated circuit die are attached to conductors of the leadframe with bond wires. Typically, the backside of the semiconductor integrated circuit die is attached to the die attach paddle of the leadframe with an adhesive such as epoxy. Over time, conductive paths may form between the semiconductor integrated circuit die and the die paddle of the leadframe. These conductive paths may be created by migration of silver from the die attach paddle of the leadframe to the backside of the integrated circuit die. Eventually this silver migration creates a connection between the backside of the semiconductor integrated circuit die and the die paddle, thus causing an electrical short therebetween.
Silver molecules from the die attach paddle can migrate over time when there is an electrical potential difference between the semiconductor integrated circuit die and the die attach paddle. This electrical potential difference is present when operating and/or standby power is applied to the semiconductor integrated circuit die. Silver migration is particularly active when the semiconductor integrated circuit die is drawing low quiescent current, e.g., during a standby mode of operation (sleep mode). Running the semiconductor integrated circuit die at a low quiescent current is necessary so that the semiconductor integrated circuit die may be brought from the standby (sleep) mode to an operating mode. Silver migration creates electrical paths between the semiconductor integrated circuit die and the die attach paddle and thereby causes high quiescent current in the semiconductor integrated circuit die to the point eventually where the circuits of the semiconductor integrated circuit die fail.
Various physical attachment configurations have been used to electrically isolate the semiconductor integrated circuit die from the die attach paddle. One such attachment configuration uses non-conductive epoxy to achieve physical attachment and electrical isolation. However, this has proven over time to be ineffective and unreliable in preventing high quiescent currents due to conductive paths between the semiconductor integrated circuit die and the die attach paddle.
Another similar but more effective approach is to screen print one or two layers of non-conductive epoxy onto the backside of an integrated circuit wafer. The integrated circuit wafer comprises a plurality of semiconductor integrated circuit dice. The screen printed epoxy is partially cured (B-stage) and is thereafter ready for singulation into individual dice. The individual integrated circuit dice are then attached to respective die attach paddles of a plurality of leadframes by heating the die attach paddles then scrubbing the non-conductive B-stage epoxy coated dice onto the heated die attach paddles. After the dice have been attached to the respective die attach paddles, they are heated until the non-conductive B-stage epoxy is hard cured (C-stage). This form of attachment does solve the silver migration problem long term, but the useful storage life of the B-stage epoxy integrated circuit wafer is only about two to four weeks.